Issue #26243: Only the level argument to zlib.compress() is keyword argument

now.  The first argument is positional-only.

Doc/library/zlib.rst

@@ -58,7 +58,7 @@ The available exception and functions in this module are:
    Raises the :exc:`error` exception if any error occurs.
 
    .. versionchanged:: 3.6
-      Keyword arguments are now supported.
+      *level* is now supported as keyword arguments.
 
 
 .. function:: compressobj(level=-1, method=DEFLATED, wbits=15, memLevel=8, strategy=Z_DEFAULT_STRATEGY[, zdict])

Lib/test/test_zlib.py

@@ -163,8 +163,10 @@ class CompressTestCase(BaseCompressTestCase, unittest.TestCase):
         self.assertEqual(zlib.decompress(x), HAMLET_SCENE)
 
     def test_keywords(self):
-        x = zlib.compress(data=HAMLET_SCENE, level=3)
+        x = zlib.compress(HAMLET_SCENE, level=3)
         self.assertEqual(zlib.decompress(x), HAMLET_SCENE)
+        with self.assertRaises(TypeError):
+            zlib.compress(data=HAMLET_SCENE, level=3)
 
     def test_speech128(self):
         # compress more data

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.6.0 alpha 3
 Library
 -------
 
+- Issue #26243: Only the level argument to zlib.compress() is keyword argument
+  now.  The first argument is positional-only.
+
 - Issue #27038: Expose the DirEntry type as os.DirEntry. Code patch by
   Jelle Zijlstra.
 

Modules/clinic/zlibmodule.c.h

@@ -3,7 +3,7 @@ preserve
 [clinic start generated code]*/
 
 PyDoc_STRVAR(zlib_compress__doc__,
-"compress($module, /, data, level=Z_DEFAULT_COMPRESSION)\n"
+"compress($module, data, /, level=Z_DEFAULT_COMPRESSION)\n"
 "--\n"
 "\n"
 "Returns a bytes object containing compressed data.\n"
@@ -23,7 +23,7 @@ static PyObject *
 zlib_compress(PyModuleDef *module, PyObject *args, PyObject *kwargs)
 {
     PyObject *return_value = NULL;
-    static char *_keywords[] = {"data", "level", NULL};
+    static char *_keywords[] = {"", "level", NULL};
     Py_buffer data = {NULL, NULL};
     int level = Z_DEFAULT_COMPRESSION;
 
@@ -460,4 +460,4 @@ exit:
 #ifndef ZLIB_COMPRESS_COPY_METHODDEF
     #define ZLIB_COMPRESS_COPY_METHODDEF
 #endif /* !defined(ZLIB_COMPRESS_COPY_METHODDEF) */
-/*[clinic end generated code: output=9bd8a093baa653b2 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=ba904dec30cc1a1a input=a9049054013a1b77]*/

Modules/zlibmodule.c

@@ -143,6 +143,7 @@ zlib.compress
 
     data: Py_buffer
         Binary data to be compressed.
+    /
     level: int(c_default="Z_DEFAULT_COMPRESSION") = Z_DEFAULT_COMPRESSION
         Compression level, in 0-9 or -1.
 
@@ -151,7 +152,7 @@ Returns a bytes object containing compressed data.
 
 static PyObject *
 zlib_compress_impl(PyModuleDef *module, Py_buffer *data, int level)
-/*[clinic end generated code: output=1b97589132b203b4 input=abed30f4fa14e213]*/
+/*[clinic end generated code: output=1b97589132b203b4 input=638d54b6315dbed3]*/
 {
     PyObject *ReturnVal = NULL;
     Byte *input, *output = NULL;